Most reactive hot-melt adhesives are high-solids, thermoplastic prepolymers having free isocyanate (NCO) contents from 1 to 5 wt. %. The prepolymers set quickly upon cooling as the prepolymer crystallizes, thereby providing good green strength. Ultimate tensile and adhesive properties are achieved as the isocyanate groups in the prepolymer react with atmospheric moisture over 1 to 7 days to provide a fully cured, thermoset adhesive. Reactive hot-melt adhesives work most effectively with porous substrates that more readily allow moisture to diffuse into the prepolymer to accomplish a cure. For some examples of reactive hot-melt adhesives, see U.S. Pat. Nos. 7,641,968 and 6,448,193 and PCT Internat. Publ. No. WO 2014/114758.
Aliphatic polyester polyols, especially those having a high degree of crystallinity, are commonly used for reactive hot-melt adhesives (see, e.g., PCT Internat. Publ. No. WO 01/40343, U.S. Publ. No. 2003/0212235, and U.S. Pat. No. 5,599,895).
Polyester polyols made from combinations of aliphatic and aromatic diacids have been described in a variety of contexts. Most of these have a relatively high proportion of the aromatic diacid (see, e.g., EP 0034919; EP 0248570; U.S. Pat. Nos. 3,804,810 and 8,334,035; and U.S. Publ. Nos. 2007/0129523 and 2010/0126664). Occasionally, scrap PET is described as the source of the aromatic diacid for making the mixed aromatic/aliphatic polyester polyol (see, e.g., EP 0248570). In some cases, the mixed aromatic/aliphatic systems are used to produce moisture-cured, hot-melt polyurethane adhesives from NCO-terminated prepolymers (see, e.g., U.S. Publ. Nos. 2010/0126664, 2010/0104831 and 2007/0129523; PCT Internat. Publ. No. WO 2014/114758; and U.S. Pat. No. 7,641,968).
Polycarbonates, such as bisphenol A polycarbonate, can be recycled. Usually, scrap polycarbonate is pulverized and melt extruded to produce polycarbonate thermoplastics. Recycled polycarbonates have apparently not been used as a raw material to modify aliphatic polyester polyols or to use such polyols to make reactive hot-melt polyurethane adhesives.
A continuing issue for reactive hot-melt adhesives is how to produce adhesives with adequate green strength and good ultimate adhesion to a variety of common substrates. Adhesion to polyolefins (e.g., polyethylene or polypropylene) is particularly challenging due to the nonpolar nature of these substrates.
Improved polyols are needed. In particular, the urethane industry needs sustainable polyols based in substantial part on recycled polymers such as polycarbonates and the practically unlimited supply of recycled polyethylene terephthalate. Polyols with high recycle content that satisfy the demanding viscosity, functionality, hydroxyl content, and other requirements of polyurethane formulators would be valuable. Of particular interest are polyols that can provide hot-melt polyurethane adhesives with good green strength and good ultimate adhesive strength when applied to paper, wood, plastic, glass, ceramic, or metal substrates.